This invention relates to reducing yellowing caused by thermal curing of polyurethane coatings that utilize a blocked isocyanate crosslinking agent.
Coating compositions can be characterized as being of the one-component type or the two-component type. In the two-component or "two-pack" system, the polymer and the cross-linking agent are combined just prior to coating. The two-component system allows for the use of highly reactive cross-linking agents, which can provide advantageous physical properties to the coating. However, the process usually requires mixing of the components in the right proportion immediately prior to coating, and can be expensive to operate and difficult to control. The one-component or "one-pack" systems offer significant advantages because the cross-linkable clearcoat coating composition can be handled as a single formulation. However, when highly reactive crosslinking agents such as polyisocyanates are used, the cross-linking agent in one-component systems must be blocked in order to prevent premature cross-linking of the clearcoat composition. The blocking group can then be unblocked under specified conditions, such as high heat, to allow the materials to crosslink so the coating can be cured. Isocyanates can be blocked, for example, with ketoximes.
Coatings that have been cross-linked with blocked isocyanate cross-linking agents also exhibit good hardness, and they are resistant to environmental etch. Generally, they suffer from severe yellowing during heat curing. Unlike coatings derived from unblocked isocyanates, where yellowing is primarily limited to aromatic polyisocyanates, coatings derived from blocked isocyanates exhibit yellowing during cure even when an aliphatic polyisocyanate is used. This problem is particularly significant when ketoxime blocking agents are used.
U.S. Pat. No. 4,369,301 discloses incorporating the hydroxy functional reaction product of hydrazine and a cyclic carbonate, which is a hydroxy functional N-amino alkyl carbamate, into a polyisocyanate to reduce yellowing. The result is disclosed as incorporating urethane groups and --NH--NH--CO--O-- groups in the polyisocyanate, indicating molecular chain extension by means of the hydroxy functional N-amino alkylcarbamate. The patent is said to be directed to two-component, thermosetting compositions. The use of hydrazide compounds having structural units according to the formula: EQU --CO--NH--NH--CO--
to reduce yellowing in one-component polyurethane coatings utilizing blocked polyisocyanates has been described in U.S. Pat. No. 5,216,078. U.S. Pat. No. 5,112,931 discloses these same structural units incorporated into blocked isocyanates. U.S. Pat. No. 4,983,739 discloses hindered amine stabilizers based on hydrazides that may be separate from the resin systems they are designed to protect.
It is an object of this invention to provide reduced yellowing to coating compositions utilizing one-component polyurethane resin systems employing blocked isocyanate cross-linking agents.